1. Field of the Invention
The present invention relates to method and apparatus for heat-processing semiconductor wafer or LCD and, more particularly, it relates to method and apparatus capable of controlling temperatures of these semiconductor wafers or LCD when these are heated up and cooled down.
2. Description of the Related Art
Various kinds of heating processes are included in the course of manufacturing various kinds of devices in which semiconductors are included. Semiconductor wafers are heated at the adhesion and baking processes in the course of photo-lithographing semiconductor integrated circuits, for example. More specifically, the semiconductor wafers are heated and their surfaces are treated with HMDS vapor at the adhesion process so as to promote the photoresist bonding performance on wafer surface. After the semiconductor wafer being coated with photoresist, which is baked at a certain temperature to remove solvent in the photoresist and to enhance the polymetric cross linking of photoresist.
As shown in FIG. 1, various kinds of processing units 1 to 6 are housed in a resist-processing system 10 to process the semiconductor wafers one by one. A sender 1 is located on the inlet side of the system 10 and a receiver 6 on the outlet side thereof. The semiconductor wafer is transferred one by one from the sender 1 to an adhesion unit 2, and adhesion-processed by the adhesion unit 2. After being heated by the adhesion unit 2, the semiconductor wafers are forcedly cooled by a cooling unit 3 and coated with photoresist in a coating unit 4. After being coated with photoresist, they are baked by a baking unit 5 and transfer to the receiver 6. They are carried from the receiver 6 to an exposure unit (not shown) through an interface (not shown) and exposed by the exposure unit located outside the system 10.
It is needed that the semiconductor wafers are forcedly cooled at a high speed by the cooling unit 3. This is because temperatures of the wafers must be accurately controlled and their surfaces must be cooled to have a certain temperature so as to uniformly coat their surfaces with resist at a next process.
In other words, the temperature of the wafer which has been processed by the conventional adhesion unit 2 depends upon its temperature obtained when the adhesion process is finished relative to it on a wafer-stage in the unit 2. This causes its temperature to variously changes and not to be kept certain. After it is adhesion-processed by the conventional unit 2, therefore, it must be cooling-processed by the unit 3. The whole of the conventional resist-processing system thus becomes large in size. In addition, a forcedly-cooling time is added to the wafer-carrying time in the cooling unit 3 in the case of the conventional system, thereby reducing the throughput of the wafers. Further, when the time during which the wafer is processed through the whole of the system is long, it causes more particles of dust and the like to adhere to the semiconductor wafer. This is not preferable from the viewpoint of quality control.
In the case of the conventional baking unit 5, the semiconductor wafer is mounted on a heating plate made of stainless steel or aluminum alloy and heated by the heating plate. A thick heating element is embedded in this conventional heating plate, the plate has a great thickness of 30 to 50 mm and its heat capacity is large accordingly. This causes the conventional heating plate not to quickly raise and lower the temperature of the wafer-stage in the baking unit 5. Particularly, quick cooling is theoretically difficult because the heating plate has a limitation in its cooling speed.
As apparent from the thermal history curve shown in FIG. 2, the temperature of the wafer-stage can be controlled only when it is kept at a baking temperature T.sub.1 but it cannot be controlled while it is being raised and cooled. The thermal history curve covering the whole of baking temperatures cannot be therefore guaranteed as desired. In addition, the heat capacity of the heating plate is large in this case. The heating and cooling response of the heating plate is thus made slow and those times D.sub.4 and D.sub.7 which are needed to raise and lower the temperature of the wafer-stage become long, respectively.
As the thermal history curve in FIG. 3 indicates, the baking unit heats the semiconductor wafer stepwise in some cases, thereby to enhance the thermal durability of the resist film formed on the wafer by cross linking polymerization. The baking unit is controlled such that the temperature of the wafer-stage remains at baking temperature T.sub.2 for a predetermined time and then at baking temperature T.sub.3 for a predetermined time. However, neither the time D.sub.5 for heating the stage from an initial value to temperature T.sub.2 nor the time D.sub.6 for heating it from temperature T.sub.2 to temperature T.sub.3 is controlled at all. These periods D.sub.5 and D.sub.6 are relatively long, and the sum of them is thus considerably long, inevitably lengthening the total baking time very much. Consequently, the throughput of the wafers is reduced. To prevent reduction of throughput, it is necessary to use a number of heating plates.
As the density of semiconductor devices is made higher and higher, resist patterns of these devices have become finer and finer. The thermal history curve (or temperature changing pattern) of the wafer-stage at the stage-heating and -cooling times which was neglected in the case of the conventional system comes to add large influence to the resolution and light-sensitivity of the photoresist. This makes it seriously necessary to develop a system capable of controlling the temperature of the wafer-stage to obtain a thermal history curve as desired at the stage-heating and -cooling times.
Particularly in the case of the conventional system, the temperature of the wafer-stage was not a controlled predetermined condition at the stage-heating and -cooling times. Even when the semiconductor wafers of the same kinds were baking-processed, therefore, the property of one wafer became different from those of the other ones, thereby damaging the reliability of the semiconductor wafers thus produced.